Begin by greasing a 9x13 inch baking dish with butter or cooking spray to prevent sticking.

In a large saucepan, melt the unsalted butter over low heat.

Add the marshmallows and stir continuously until fully melted and smooth.

Stir in the pumpkin puree and pumpkin pie spice until well combined.

Remove the saucepan from the heat and add the Rice Krispies cereal. Stir until all the cereal is evenly coated with the marshmallow mixture.

If desired, fold in chopped nuts or chocolate chips for extra flavor and crunch.

Transfer the mixture to the prepared baking dish, pressing it down firmly with a spatula to create an even layer.

Allow the mixture to cool at room temperature for about 30 minutes before cutting into squares.